Advanced energy storage systems such as lithium-ion batteries are important approaches to mitigate energy shortage and global climate warming issues that the world is currently facing. High power and high energy density are essential to batteries for applications in electric vehicles, stationary energy storage systems for solar and wind energy, as well as smart grids. Because conventional lithium-ion batteries are inadequate to meet these needs, advanced materials with high capacity and fast charge-discharge capability are desirable for next generation lithium-ion batteries. Titanium dioxide (TiO2) and various polymorphs (anatase, rutile, and TiO2-B (bronze)) have been widely investigated as lithium-ion battery anode materials, due to their advantages in terms of cost, safety and rate capability. In particular, the polymorph of TiO2-B has shown a favorable channel structure for lithium mobility, which results in fast charge-discharge capability of a lithium cell. It has been identified that the lithium intercalation in TiO2-B features a pseudocapacitive process, rather than the solid-state diffusion process observed for anatase and rutile. Theoretical studies have uncovered that this pseudocapacitive behavior originates from the unique sites and energetics of lithium absorption and diffusion in TiO2-B structure. As a result, TiO2-B nanoparticles, nanotubes, nanowires, and nanoribbons have been reported as anode materials with good rate performance for lithium-ion batteries. These nanomaterials displayed attractive battery performance; however, they also have some disadvantages, e.g., poor electronic conduction network due to aggregation of nanopowders, loss of particle connection during cycling, and low packing density.
Mesoporous materials (LiFePO4 and TiO2) with micrometer-sized particles were found to be able to overcome these shortcomings, yet still maintain the advantages of nanomaterials. The properties of mesoporous materials ensure high contact area between electrolyte and electrode, short diffusion distances for Li+ transport, and good accommodation of strain during cycling. The general concern for mesoporous materials is the long transport distance of electrons in micrometer sized particles. Conductive carbon and RuO2 coatings have thus been employed to improve the high rate performance of lithium storage in mesoporous TiO2 materials.
However, even without conductive coatings, mesoporous anatase materials have shown high capacity and good rate performance. It is believed that the sintered nanograins in mesoporous materials could form a facile electronic transport path because of the accumulation of electrons at the grain-grain interface. On the other hand, micrometer sized materials with a spherical morphology are actually the optimal material morphology in conventional electrode fabrication art, because microspheres have high packing density and good particle mobility to form a compact electrode layer. These features are beneficial to attain high volumetric energy and power density as well as uniform electrode layers. Therefore, it is highly desirable to develop a mesoporous electrode material, such as a mesoporous TiO2 electrode that could combine the advantages of the TiO2-B polymorph, mesoporous structure, and spherical morphology, that has a high capacity and fast charge-discharge capabilities. Additionally, it is desirable to develop a simplified and economic approach to form the mesoporous electrode material.